Can Helicopters fly inverted? (Merged threads) [Archive]

CRAN

15th May 2002, 14:13

Twin 1,

Yes, theoretically helicopters can fly inverted for sustained periods, but lot's but lot's of systems would have to be designed differently from the outset to accomplish this. Such as - hub would need to produce massive control moments to counter static instability, you would need a high performance stability augmentation system, the fuel system would need to be pressurised & and the engines modified to be intolerant to attitude. To mention but a few!

Incidentally, on the subject of negative pitch and coning - the Lynx has some negative collective pitch travel and consequent negative coning to pin it to the rolling decks of ships in rough seas prior to being lashed down.

What were you intending to do for your project and where are you studying?

CRAN

Off road

15th May 2002, 14:51

I think this would be an interesting question for Nick Lappos.

I wonder whether the Commanche can fly inverted for a sustained period?

Fly safe;)

luck is what happens when preparation meets opportunity

sling load

15th May 2002, 15:11

for a given airspeed and collective pitch setting in normal flight, the whole rotor is rigged with the fuselague hanging beneath it, in the theoretical case of inverted flight, you would need substantial negative pitch over what is required for normal flight, plus as cran said, a stab aug system that would allow a fuselague to remain above it IMHO.

That in itself would be something probably more expensive than the helo. Yes, the RCs can do it, but I would like to see the hp/weight ratio compared to a helo.

It would be a most unnatural thing to lower the collective when approaching the inverted, ouch!

Robbo Jock

15th May 2002, 15:41

I've seen pictures of RC helicopters inverted. I've often wondered how they manage it.

Normal situation, disk-a-spinning, body hanging beneath it, swinging gently in the hover. Disk tilts forward and starts sliding in that direction The body, subject to drag and inertia, slides slowly backwards, rotating about the pivot - the rotor hub. At some point all the forces will end up in equilibrium (drag, weight of body, horizontal and vertical components of total reaction) and the body will end up hanging at some angle, swinging gently in relation to the disk. This angle can be varied by adding horizontal stabilisers and such, but basically the body hangs back from the disk and is held from moving further back by its weight. A stable situation.

In the inverted state, the disk is spinning away and the body is 'hanging' above it. Balancing a broomstick vertically on the end of one's finger springs to mind. This is an innately unstable state, any movement is sure to be divergent. Assume the CoG is absolutely vertically above the hub and we're stable. Tilt the disk forwards and the disk will start sliding in that direction. The body, subject to drag and inertia, slides slowly backwards, rotating about the pivot. We're now balancing that broomstick whilst standing on a skateboard that's started downhill. (I know, as Helicopter pilots, this is all in a days work and part of the fun !) But once the CoG has moved from vertically above the pivot point, there's nothing to stop it continuing to move. Weight's still pulling it back, drag's still pulling it back, there's nowt to hold it up. NOT a stable situation.

Forget coning, forget boom clearance, forget pitch, if the CoG is above the pivot, it's unstable. How do even RC helicopters manage to control this ? Are their hubs absolutely rigid in-plane ?

widgeon

15th May 2002, 15:49

Reminds we what one wag posted about best ROC for a helicopter could be obtained by flying inverted and autorotating up . I imagine biggest prob would be keeping fuel flowing to the engines unless you had a feed at the top and bottom of the tank.

CRAN

15th May 2002, 15:50

Robbo,

My understanding is that they do it in the following manner:

Firstly, the unstable moment created by the CG being out of line with the pivot point is balanced by hub moments. This is inherently dependant on the pilot/RC Operator having the reactions to correct the divergence before the body rotates to such a degree that the maximum moment achievable by the hub is exceeded and the aircraft crashes.

I agree with your analogy with the up-turned pendulum, this configuration is statically unstable - but that's not to say it couldn't be designed to work if the need arose. Blades don't need to be rigid and theoretically any hub will work though the more control power the better!

:)

CRAN

heedm

15th May 2002, 17:45

I guess to be pedantic, we're talking about sustained LEVEL inverted flight. Or better yet, can a helicopter maintain -gz?

Theoretically has been discussed, but practically I'd like to hear ideas why we'd want to do this. Other than airshows and kicks, the only purpose I can think of would be an agressive bunt for someone getting attacked. Maybe the Comanche can do this?

Interesting aside. 1996 International Helicopter Championships in Salem, Oregon (great time, outstanding hospitality). National title holding RC Helicopter aerobatics guy put on a show that I still can't figure out. Inverted flight quite common, but vertical dives with extremely rapid yaw rates, pointing nose down at ~10' AGL then pushing into an inverted hover at inches. WOW. Slow level flight ~6' AGL starts pitching 360 degrees while maintaining altitude and slow forward flight. No idea how this one works, must be some sort of illusion thrown in with exceptional flying skill.

Second show of day was cut short when 6" inverted hover turned into 0" inverted hover and $5000 helicopter turned into 5000 pieces. Talked to the guy afterwards. His attitude "part of the job". My attitude, "I'm not flying real helicopter inverted". especially at 6".

Dave Jackson

15th May 2002, 23:00

Twin 1,

A helicopter with a really really rigid rotor located below the 'fuselage', is extremely stable. In fact, it is too stable. As the forward speed increases the drag of the fuselage resists the necessary forward tip of the disk.

Two examples are:- http://www.flying-platform.com/ and http://www.hiller.org/exhibits/online-exhibits/flying-platform/flying-platform.html

RW-1 & heedm

>When discussing inverted forward flight, or even an inverted hover for full scale, I also tend to wonder "Why?" <

The UniCopter (http://www.unicopter.com/unicopter.html) is intended to fly inverted. This feature is for use during combat. When attacking, the helicopter performs barrel rolls. This causes the enemies to laugh so hard that they can't hold their weapons steady. :eek: :eek: :eek:

Nick Lappos

16th May 2002, 00:55

Inverted flight in real helicopters is a neat parlor trick, and many modern helos are capable of brief encounters with negative G, but none are capable of continuous inverted flight due to a variety of reasons. Having flown and demonstrated negative G and inverted maneuvers, here is my read on it. (Caution, in many helicopters, negative G can result in loss of control or rotor failure) :

1) Engine and fuel systems are not designed to maintain proper fuel and oil flows, so the fuel pressure and oil pressure lights come on almost immediately. If positive G not restored in a few seconds, fuel or oil starvation will result in engine shutdown or internal failure.

2) Main rotor control is the sum of the thrust and the moment the rotor produces. As thrust is reduced, control effectiveness diminishes, to nil in many rotor designs. If there is sufficient hinge offset (the distance of the flapping hinge from the mast, expressed as a percentage of the rotor radius) the rotor can still be effective at negative G. For high offset rotors (such as the BO-105), -1 G is possible. For the Black Hawk and S-76, about -.5 to -.6 G is all you can reliably get to. I used to demonstrate -.5 G with snappy roll reversals in the armed S-76 and in Black Hawks.

For teetering rotors, like the older Bell family, the Tilt Rotors and the Robinson, fill out all your personal paperwork and settle all debts before trying anything like low G, because you won't have to worry about the landing.

Comanche can get very negative, because it has a high offset main rotor, but its fuel system is not designed for it. There really is no mission need to perform extremely low G for any length of time.

Dave Jackson

16th May 2002, 02:43

As mentioned, the idea of flying inverted is probably only a hypothetical situation, but it does raise interesting aerodynamic considerations.

I believe (subject to correction) that in right-side-up forward flight the speed stability is achieved by the positive pitch moment of the rotor (flap-back) being slightly more dominant than the negative pitch moment of the fuselage.

Assuming that the rotor is capable of providing negative pitch and that the helicopter is flying inverted, will not the pitch moment of the rotor (flap- back???) and that of the fuselage both be positive (in respect to the horizon, not the helicopter). This seems to imply that there will be excessive speed stability and that the maximum forward speed will be very limited.

Nick Lappos

16th May 2002, 04:09

In trimmed upside down flight, the rotor would behave exactly as it now does. At trimmed speed, an acceleration would require forward stick, and the rotor would back-flap in a stable manner. The fuselage would apply a resisting moment from the horizontal tail, as well.

I think the belief that the CG effects of the body's mass are dominent is not accurate, and overplays the effects of cg on the basic stability of the aircraft. In most helicopters in normal flight, the CG can be a positive or negative stability term, depending on the relationship to the mast. At aft cg in normal rightside-up flight, the mass is also an unstable term (an increase in G would cause an increase in nose up moment from the cg). This is a real effect, has the same magnitude whether upside down or not, and is easily accounted for in the basic design of the aircraft. In upside down flight, the aft cg would still be de-stabilizing, and a forward cg would be stabilizing.

Twin 1

16th May 2002, 09:14

Cheers Guys,

There are many good points raised, some of which I barely grasp, I will endeavour to further my understanding. However from the replies I can see that this is an interesting subject.

I am considering a final year project investigating the aerodynamics of inverted flight and inverted helo flight.
If I have any problems I now know where to come for answers! ;)

As to the question Why bother?

No idea, I just thought why has it not been done, is it possible?

Question: If this helo was built, would you fly it???????????:D

Thanks for the replies,

Twin

Dave Jackson

16th May 2002, 18:34

Nick,

For the fun of a hypothetical discussion;

>In trimmed upside down flight, the rotor would behave exactly as it now does.<

I believe you are saying that if there was any coning it will be upward away from the earth, but from the inverted pilot's perspective, it is 'downward' toward the fuselage. In other words, from a remote observer's perspective, the only thing unusual about the rotor disk is that the mast is above it.

>At trimmed speed, an acceleration would require forward stick, and the rotor would back-flap in a stable manner. <

IMHO acceleration will require aft stick, since the thrust from the rotor disk is now in the opposite direction (negative pitch).

I will now bend over and assume the 'Lu' position. :D
______________________________

Twin 1,

>As to the question Why bother? <

You may have come up with an industrial size weed-eater. :rolleyes:

Aesir

16th May 2002, 19:52

Ray Prouty says a upside down helo can be built but it has abolutely no practical purpose so why bother!

Nick Lappos

17th May 2002, 01:27

Dave,

You are right, I had it backwards, the upside down helo would need to have the pilot pull the stick back to make the rotor tilt more nose down! It hurts to think too hard about it!

Nick

heedm

17th May 2002, 03:44

Oh it hurts is an understatement.

You're upside down level forward flight. Pull stick back, rotor disc tilts so the total thrust is now further forward. You now have a moment about the c of g that will pitch the nose down....errr up...you see more sky (or is it less ground). Anyway, to stop that pitching motion you add forward cyclic. So do you slow down now??

HeLP!! Where did I go wrong?

RW-1

17th May 2002, 13:59

Yes, well .... now you have a taste of the mental gymnastics us model heli pilots make when we invert the machine :D

By the way, setup of the machine are just as neat, I run what we call a 3D curve on my radio.

One has throttle and coll pitch curves on the radio, typically we set them to maintain RPM throughout the range of collective.

But for 3D, I set my collective pitch from max inverted pitch at low end to max at high end, zero in the middle, and the throttle now is in a "V" curve, meaning full throttle at both ends.

Can make for some of the more interesting maneuvers some of you have seen models do.

Another interesting thing is that we can also do inverted auto's, flipping to upright near the end and flaring/landing. :D
(not that I'm doing them yet, but it can be done)

ShyTorque

17th May 2002, 19:01

I understand (as taught by RAF CFS) that a conventional heli in the hover has static stability (pendulum effect) and dynamic instability (flapback causes overshoot of original attitude and divergence)).

Invert the aircraft and you have the opposite.

A 1960s design took advantage of this (I have seen pictures of it but I can't recall its name), it was a simple manned platform with the rotor below the C of G. It was the closest I've seen to a hover board! The pilot stood on the platform and fired it up, holding onto a handrail arrangement on top of the engine. It was controlled by weight shift and it would hover just about hands off. Not a good device to abandon ship from though :D

I think it never went into production because it was too stable to be a sensible mode of transport.

Lu Zuckerman

17th May 2002, 19:51

To: Shy T

If you are referring to the “Flying manhole cover” developed by Hiller it never went into production because of instability rather than being too stable. Air was drawn down into a Venturi duct and passed through two counter rotating props, which accelerated the flow. When the pilot would lean in any given direction the craft would fly in that direction. With the Venturi duct tipped from the vertical axis the air would flow into the duct faster on the leading edge (direction being flown) and there would be an increase in lift due to the aerodynamic shape of the duct. This differential of lift would cause the craft to return to the vertical and the pilot would have to start all over. It is similar in some ways to flap / blowback in which case the pilot must add more forward cyclic.

:D

ShyTorque

17th May 2002, 20:09

Lu,

Thanks for that; I'm not sure what I am thinking about was a ducted fan...

Aha!

I just noticed Dave J's links at the top of this page. Having followed them, what I remember seeing was more like the "Flying Platform" than the Hiller, although the former machine is obviously a much more recent thing.

The Hiller design does look unstable; seems to have quite a small diameter rotor and probably a high C of G. Presumably, having a duct around the fan would reduce the flapback effect quite considerably.:)

Nick Lappos

19th May 2002, 00:37

Shy Torque,
You are quite correct, the duct of a ducted fan isolates the fan from the free stream flow, so that there is almost no back flap. This is the reason why Comanche can do the snap turn at 100 knots - its fan does not produce any strong flapping forces or moments while at large sideslips. A typical tail rotor would produce very high moments and flapping forces if subjected to the same maneuver.

For the same reason, a fenestron or ducted fan is a poor device for yaw stability at high speed, The fan simply does not produce any change in thrust for sideslip variations. That is why fenestrons reqyure such great vertical fin areas (and endplates).

Nick

teeteringhead

19th May 2002, 07:39

To elaborate on ShyTorque's reference to CFS(H) training; the C of G arguments about stability above are mistaken. (I only speak of real helicopters, not RC models, of which I know nothing).
In an unstabilised helicopter (no autopilot, autostab, ASE), the fact that the C of G is below the disc does make it dynamically UNstable in the hover.
Consider: an unstabilised conventional helo in a steady hover diverges to one side, thus creating a relative airflow over the disc. Disc flaps away from the movement, creating a correcting movement of the rotor disc. However, (isn't there always a "however" in helicopter P of F), the pendosity of the body then tilts the disc even further, thereby OVER correcting, and leading to movement in the other direction, beyond the original hover point. All this WITHOUT cyclic movement, rapidly leading to the undamped phugoid characteristic of dynamic instability. Which is why hovering an unstabilised helo is like balancing a stick on your finger. Put the C of G ABOVE the disc, and you get dynamic stability - not sure how you get on or off though!! And certainly we were taught that gave stability to the "flying platform" of the 50s/60s
That's the CFS (H) version anyway, as taught to generations of military helo pilots, by such luminaries as Chris Tinkler, Ray Lawrence, Ron Jones and Mike Ramshaw - where are they all now?

ShyTorque

19th May 2002, 08:36

Dave Jackson

19th May 2002, 18:21

Here's a web page with lots of neat information. http://www.unicopter.com/7up.gif

http://www.howtoadvice.com/Sky-High

Twin 1,

Perhaps your inverted rotor may be an excellent project. The above article is interesting and it does suggest that there are practical applications for this configuration.

ShyTorque,

The following picture, from the above site, may be the craft that you are thinking of. http://www.howtoadvice.com/Image/Photo/McCartyHeli-Vector.jpg. [The link does not appear to work. It's an enlargement of the fifth picture in the above web page.] Prouty says that this craft "had hingeless blades with high inherent hub stiffness, and consequently its stability did not benefit significantly from its unique rotor location." This may be aerodynamically similar to the ducted fan and the comments by Nick.

teeteringhead,

A teetering head may be what you are referring to. If the head is rigid, as mentioned above, then its rotational inertia should provide some dynamic stability, in dampening the oscillations. Just a thought.

Lu,

Were any of your distant relatives in aeronautics? This web site describes the following patent by a Mr. Zimmerman. http://www.howtoadvice.com/Image/Diagram/Zimmerman1Lg.jpg :)

Lu Zuckerman

19th May 2002, 20:40

To: Dave Jackson

Sorry Dave the only thing I have in common with Mr. Zimmerman is that my last name begins with Z and ends in man. However I did work with a couple of people while in service that invented Pop-Out- floats, Friction locks for flight controls, The hydraulic hoist and Hands off stable flight for helicopters among several other things on helicopters to include the rescue basket used in SAR.

My only contribution to helicopters was in the maintenance arena which allowed the rigging of helicopters without the use of a universal propeller protractor and having to level the helicopter. And, a device that allowed the lubrication of rod ends without removing the control linkage from the helicopter.

ShyTorque

19th May 2002, 21:42

Dave,

Ah yes, the Amphibious Heli Vector. The first aircraft to be fitted with airbags? Invented by that famous inventor chap McCarty, latterly known as "Shorty". :eek:

His demise came whilst trying to carry out a one-man rotors running refuel whilst cattle mustering to avoid cycles on the engine....I guess he just forgot.....:D

ShyTorque

19th May 2002, 21:57

Dave,

on a more serious note..

My understanding is that an inverted rotor system will only benefit from enhanced stability if it can flap back. A "rigid" rotor would roll laterally instead (as Cierva found out until he used flapping hinges on his autogiros). Two contra-rotating rigid rotors presumably would not exhibit either effect, as per a ducted fan.

Dave Jackson

20th May 2002, 01:48

ShyTorque,

You expect a serious response after your previous post ???:D???
_________________________

I agree with all you say.

The drag of the pilot will tend to pitch the 'nose' of the craft up. The reason that Lu's long lost relative has wings and headgear is probably to enhance this parasitic drag.

As you mentioned, ducting should eliminate any potential nose up pitching resulting from the rotors.

Your points about un-ducted rotors make sense, if the craft's rotors have 'absolute' rigidity, plus no coning. As you said, rolling is then prevented because of the two counter rotating rotors.

If we assume that the rotors do not have 'absolute' rigidity, then there will be a small amount of flap back taking place.

Nick and Roberto Celi have already corrected me on something related to the following and Nick may have cause to do so again.:) If the rotors have coning as well, then there will be some additional lifting taking place at the front. This is because the leading blade will have a slightly higher angle of attack then the retreating blade.

All three things probably contribute to the nose up pitching and thus the static stability of the craft. The greater the rigidity, the more that the coning will cause nose up. The less the rigidity, the more that flap back will cause nose up.
_____________________

If you buy this, then there's a bridge in Brooklyn we can talk about. :D :D

Twin 1

20th May 2002, 08:20

Dave,

Cheers for that, some interesting reading there. :D

All,

Thanks for the replies! :p

If anyone could recommend a good web-site to understand the basic principles of the helicopter I would be most grateful.

Thanks Again,

Twin

Dave Jackson

20th May 2002, 16:51

Twin 1,

This may be of some help: http://www.unicopter.com/B285.html
_____________________

>"2 heads are better than 1"< :eek:
Two questions;
Are you referring to rotorheads? ;) May I plagiarize this, occasionally? :) :D:D:D

zoink

24th Jul 2004, 19:12

At the farnborough show today during the Apache display where it was doing loops, the thought came to me.

When a helicopter is inverted during a loop, is it in controlled flight? In a loop a helicopter will fly from normal to inverted to normal flying position so is the inverted stage an controlled section of flight or is physics just moving the a/c back to an position where the pilot is in control?

By controlled i mean that the pilot can make positive inputs to the a/c as they can do in the normal flying position

I ask the question because i cannot see how the rotors would work in the inverted position...

(my excuse for the weired question is the sun... i'm a nice shade of pink now after today :) )

Thanks in advance!!
z...

Slartibardfast

24th Jul 2004, 19:21

Getting inverted in a helicopter is simply a matter of inertia - you make the appropriate input and wait! It is most definately not contollable whilst the thing is inverted, in fact some very curious things can happen if you try! Most helicopters can do the manoevre but the limiting factor tends to be the oil in the various sumps - It starts off in the right place but ends up somewhere else! so as you roll out the CWP lights up like a xmas tree.

NickLappos

24th Jul 2004, 19:32

The issue is not the attitude of the aircraft, but rather its full maneuver state. You picture it upside down, and the rotors now having to lift upside down, but that is not what happens, in most cases.

In the loops and split S maneuvers we do, the aircraft is continuously developing some positive load factor with the rotor pulling it toward the earth, so that the pilot feels normal in his seat, and a glass of water on the dash would not spill. They can be done in most helos, but with tiny margins for error in many.

Two exceptions are the BO-105 and Comanche., Both have very controllable rotors, and can develop significant negative lift. In films, I have seen the BO pushed over to 135 degrees nose down and then just pulled back up to level. This surely created lots of negative G. These two helos come closest to your concept of true, trimmed upside down flight.

Of course, control is only part of the issue. Without inverted fuel and hydraulic systems, these helos would get very quiet after a few seconds upside down!

A D ENUFF

24th Jul 2004, 20:04

NL

Seem to remember a few years back seeing a video clip of good ol' Charlie Zimmerman holding an inverted hover in a BO 105 for at least 7 or 8 seconds which would indicate a certain degree of control in my book. That said.....what he couldn't do with that aircraft probably wasn't worth talking about.

Gather he was stopped from displaying !!!!! Bloody good pilot but must've been as mad as a fish to pull off some of those manoeuvres.

P.S. As an ex display pilot I have looped a helicopter ( properly )several times. ( Not a 105 ) and would tend to agree with Slartibardfast that once you've plucked up the b***s, commited yourself and adopted the necessary attitude the aircraft will invariably fly itself around.

zoink

24th Jul 2004, 20:31

thanks folks...

you've got to love this inertia thingy.. sure does make for a good display!!! :)

z..

tecpilot

24th Jul 2004, 21:33

One of the first things i could see from the pilot seat in a BO-105 was a 110 degrees dive to demonstrate the possibilities of my favourite ship up to date. I will never forget the first look at the Attitude indicator going into the deep, deep, deep brown... " (really i looked at the instrument, why i couldn't believe the incredible dive angle) The instructor came calm with the words "... look to the mast bending moment, allways stay in the green.... " :)
I will admit now, i was shocked!!! I had flown up to this point some really different types of helicopters but it wasn't to imagine what the small beast could do! Ok, some 100 hp and a little bit more cabin room needs the "pilotship", but thats impossible due to some constructional and CG problems.

The hydraulics and the fuel system are able to work proper on negative Gs. They are full pressurized and closed.

Aerobatics are prohibited due to some simple reasons.

1. Most pilots are not aerobatic experienced. That could easy kill you! The green area is small on some situations. In the described dive you have only round about 6-8 sec from a IAS 0kt to bring the ship back to level or you will exceed the VNE with all the problems in aerodynamic and solidity.
2. The a/c will be overstressed, especially with the rigid rotor. Such a rotor brings the stress and vibrations direct to the fuselage with a lot of cracks as the result .
3. You lost all your insurance and may be your job.
4. The a/c is not certified for such "flights".

Thats the reason why EC pilots will today never show (such) aerobatics on official dates. The German Army shows sometimes such procedures, but only very limited to special trained pilots and they have problems with the cracks and other damages. No civil operator will pay such bills.

cyclicpushover

2nd Jun 2005, 10:38

Can anyone tell me why some helicopters are capable of inverted flight (apache, lynx, bk117?) and others are not. My guess is a teetering head would be susceptible to mast bumping (lowG) but, apart from acrobatics prohibited and carburetor problems is there more involved? And can it be sustained or is it short lived.

NickLappos

2nd Jun 2005, 11:01

The concept of hinge offset is worth exploring for the real answer here. If the rotor has the ability to develop control at zero g, it has high hinge offset, and can be maneuvered more. Teetering rotors are the worst kind for maneuvering, for that reason, as they have zero hinge offset.

See this discussion, with a good illustration half way down:
http://www.w3mh.co.uk/articles/html/csm9-11.htm

glyn thrash

2nd Jun 2005, 11:16

ftp://helifreak:[email protected]/ChattiMattBotosGo-Off5-2005.wmv

like this??:O

ShyTorque

2nd Jun 2005, 14:38

There is a site showing a model heli landing on the ceiling amongst other amazing stunt flying. Look on Google for "rc helicopter videos" and take a look at the Helihobby.com site.

cyclicpushover

3rd Jun 2005, 10:17

Thanx for your replys guys, liked the vid glyn thrash & yes like that!
If teetering rotors are the worst for manouvering then is a rigid system the best?What about fully articulated systems.
None of my texts cover this subject.Does anyone know of texts that cover inverted flight in choppers?

MightyGem

5th Jun 2005, 08:00

why some helicopters are capable of inverted flight
Although some can roll and loop, helicopters cannot fly inverted in the fixed wing sense, ie they can't fly straight and level upside down. Only model helicopters can do that.

TheFlyingSquirrel

5th Jun 2005, 14:05

The thread I made a while ago had some great upside down replies and Silsoe Sid abusing public property !! ( lol )

CLICK HERE (http://www.pprune.org/forums/showthread.php?s=&threadid=169589&perpage=15&pagenumber=1)

TFS

widgeon

5th Jun 2005, 14:41

here is link to accident report guy went inverted in 222 and lived to tell the tale

http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001212X20769&key=1
THere should also be a factual report but my pdf reader is acting up. excerpt from factual report

"... severe nose high climb attitude. The cyclic stick harded over to the full aft position. I
was unable to move any of the flight controls except the pedals. The aircraft continued over, now
upside down and diving towards the ground, still at cruise power. Unable to move the cyclic, the
aircraft continued this inside loop type of attitude and started up again. Until this point I
believe I was trying to manipulate the flight controls normally with one hand on the collective,
and my right on the cyclic. I was not strong enough so I tried moving the cyclic with both hands.
As about this the cyclic started moving on it's own, and the fight now was trying to keep the
controls centered. The aircraft was completely out of control with me having minimal impact on
adjusting the flight controls."

"I either told the paramedic or he on his own initiative lowered the collective using both hands.
The aircraft then was diving at the ground and it seemed like I was more able to keep it somewhat
in a level attitude. I flared with cyclic to arrest the rate of decent and then we ballooned up in
pitch. I was trying to place the aircraft in a parking lot when we did this pitch-up. As we
climbed up again I saw power lines to my front with a road beyond that had enough room to set the
aircraft down. As I tried to raise the collective to go over these wires, the rotor bled off with
the low rotor audio and lights. I realized the aircraft was not going to clear the wires, so I
jammed full left pedal and placed the helicopter on the roof of a two-story building immediately to
my left."

slowrotor

5th Jun 2005, 18:49

RC models have no teetering hinge normally, so the head is rigid (hingeless) except for the blade attach bolt that also serves as a lead lag hinge. The collective pitch is set for several degrees of negative pitch in aerobatic models, this allows sustained inverted flight.
For a full size helo the collective lower stop is set near 0 degrees for autorotation. The 5 degrees or so of negative pitch required for sustained inverted flight would be a problem for the pilot in an autorotation emergency. Just one of the many problems to consider.

cyclicpushover

7th Jun 2005, 08:12

Thats answered some of my questions, I didn't realise that thread was there TFS, thanks.
Great pics of gazelle & aussie squirrell
A must see is Discovery Channel Extreme Choppers on DVD,with umpteen barrel rolls in a lynx.

ShyTorque

7th Jun 2005, 10:02

The Navy Lynx is capable of some negative pitch, I believe.

MightyGem

7th Jun 2005, 20:51

The Navy Lynx is capable of some negative pitch, I believe
Even when flying lynx I was never really sure about this. The correct term is "Sub minimum pitch", ie the ability to go below the normal minimum blade angle with the collective fully down when on the ground. Whether this actually went below zero always seemed to be a question. For the non lynx people, you have to physically push the collective down through a restriction to get onto sub-min.

Farmer 1

8th Jun 2005, 07:35

Widgeon - that's the stuff of nightmares.

We used to operate BO105's, and I seem to remember we had one which had two degrees of negative pitch. It's a long time ago, and I've a memory like a whatsit, but there were certainly no special procedures for applying negative pitch.

My own experience of negative g, again in a BO105, was while enjoying a flight over the Western Isles. I flew into a terrific downdraught, and became aware of a sudden vast increase in pressure on the shoulder harness, my feet flying off the pedals, clouds of dust rising from the floor, oil pressure warning lights coming on, with the pressures indicating zero, and all the while the passenger pushing up on the cockpit roof as hard as he could, barely managing to suppress a scream.

It probably didn't last more than a second or so, but I'm sure it shortened my life by a few years. My thought at the time was, "Of all the helicopter types in the world, thank God I'm in a Boelkow."

spinwing

8th Jun 2005, 13:16

"Boelkow Uber Alles!" .... even the birds are jealous!

gotta luv that "slippery lil sucker" ...

Ahhh "The earth above and the sky below" what! ......?????

;) :ok: :ok: :ok:

Head Turner

9th Jun 2005, 13:06

To those who fly in UK, we fly inverted all the time compared to those in Australia - and visa versa.

The question is worthy of technical debate but as often stressed - why would anyone wish to build a helicopter (mega, mega bucks) with an ability that would not be rational.
It's almost similar to asking why can't all boats be fitted with wheels so that they could operate on land. Or why can't all airplanes land on water and submerge.

Bronx

9th Jun 2005, 20:03

Or why can't all airplanes land on water and submerge.

Most of em do, and real quick if they ain't got floats. ;)

Brilliant Stuff

17th Sep 2005, 08:22

My little brain tells me flying inverted for longer than a brief period in a Helicopter is not possible due to the rotors hitting the tailboom and the problem of the rotors producing lift the wrong way to keep said helicopter flying for long, but since the Lynx has got such rigid rotor's I am not 100% sure, could someone help me out and confirm my simple suspicion?

Above Datums

17th Sep 2005, 10:49

Voodo mate! pure and simple :p

Thomas coupling

17th Sep 2005, 15:42

It's confirmed...your brain is very very little.

3top

17th Sep 2005, 19:10

Long shot:

a) I have NO idea about brain sizes!
b) IF we consider RC-helicopters as bonafide helicopters (they fly wit the same pricipals!), then you can see the Acrobatic versions fly upside down as long as the RC-pilot wishes!!
c) DON'T try it in a full-size!

3top
:D :D

Dave_Jackson

17th Sep 2005, 19:15

Brilliant Stuff,

In the near future, flying inverted in a helicopter will be just as easy as flying inverted in a fixed-wing aircraft.

'Absolutely' Rigid Rotors (http://www.unicopter.com/0815.html ), plus the negative blade pitch that is required for Reverse Velocity Utilization (http://www.unicopter.com/1108.html) will make this possible.

___________

Now back to building the machine (http://www.unicopter.com/A114.html), that will build the blade, that will fly inverted. :O

widgeon

17th Sep 2005, 23:31

what us the max negative that the lynx is normally set up for ? , the only time you would need negative in normal ops I would think is when you are landing on a moving deck .

Aesir

17th Sep 2005, 23:48

Well sure a helicopter could be desgined to fly inverted. But why? What would be the practical application?

Ascend Charlie

18th Sep 2005, 05:24

Dave Jackson, after you have solved the problems of the blades, rotor head (there will be some serious forces hanging around the blade attachments!!) and negative collective, don't forget to redesign the engine.

It will need an inverted flight sump, to keep oil going to the oil pump, different scavenge system because the oil will no longer drain to the bottom, and an inverted fuel system to again keep the pump supplied with fuel as it sloshes from side to side and to the top.

Ensure the aircraft gets vacuumed after every flight - get zero or negative g and all the dust and grit comes up off the floor.

Design map pockets that are very tight, to stop them coming loose. Buy a pile of new maps, because now the pilots will tear them pulling them out, and ruin them when stuffing them back in. Forget designing inverted flight coffee cup holders, I think Lockheed made one some years ago for a Hercules, and each one cost $20k.

Add to that the coffee percolator that will keep going after a thermonuclear magnetic pulse, and I think we are complete.:8

DennisK

18th Sep 2005, 10:17

Re the 'Inverted Flying' thread.

As a display pilot, can I put in a few pennorth, but only dealing with the smaller machines I fly. (PS Wish someone would offer me a go in a Lynx/Chinook)

Even the humble Schweizer 300 model will invert for a few seconds. (Just pop into my Shoreham office for a peek at the photos if your'e a non believer) and ditto the the Enstrom 280/480, MD 500/Gazelle et al.

What use is any of it.?

I've asked myself that question too many times, and have been somewhat accusingly asked it by some highly experienced guys. I only see it as a problem, if you feel it brings a swashbuckling attitude to our industry, something I constantly endeavour to avoid. But on the positive side, it DOES BRING HELI SALES, as I recall, oh so well in the early 1970's when the world was knocking the newcomer Enstrom.

I copied the great American display pilot, Mike Meger and managed to produce a display routine good enough for Farnborough, and all I can say is it put the marque on the map and subsequently I sold 138 Enstrom 28/280 Sharks in a ten year period.

At my end of the industry we need to constantly bring in the punters if we are to thrive, even survive ! And display flying does that. I've lost count of how many times, I've been approached at air shows with a worthwhile enquiry.

Of course, the articulated M/R head can't accept a smell of negative G, and to make sure of that, I leave my pace notes on the co pilots seat, and the second headset on its hook. If they move, I wake up!.

The carrier ops machines need the negative pitch to keep the skids/wheels in deck contact on a heaving ship of course, and as has been said here, the models don't seem to care which way up they are. Just a week ago, at a Draycott Farm venue, I watched Paul Heckles display his aerobatic skills at night !!!! Illuminated M/R blades and horizontal bar through the airframe. An awesome show when he spent as much time inverted as with the M/R disc the right way up.

In the 1986 World Championships at Cranfield, the German factory pilot, Charlie Zimmerman, (where are you now Charlie?) flew the B105. During his display routine, Charlie inverted the ship, entered a descending flat spin, before cycling out to level flight! Not surprising he was awarded the first place. Another German, Herman Fuchs, repeated the manoeuvre and got second.

I'm not sure I should invite any of you to 'try this at home,' but all that is needed to be inverted in the loop, is the right 'gate speed' for type, then assist the normal high speed flap back in the climb with aft cyclic, look over the shoulder for a horizontal horizon and with a quick glance at the instruments for a minimum height and manoeuvre speed on the dials, and if satis, progressively lower collective to its lowest position as the cyclic is pulled through aft. On non-correlated pistons, the Rrpm needs to be monitored as the extra G comes on in the descent and pull out.

In the USA, I've seen. the MD pilots use the manoeuvre many times for air show and publicity purposes, but again would widen my ears to any of you who had views in the oppposite camp on such flying.

As I retire from display work after 1200 practice routines and public shows, I'd dearly like to see some youngsters come along and progress things. I know for sure, there are so many more show manoeuvres to be discovered than I have produced over the years. Subject always to the maker's Flight Manual limitations. But guys, a CAA approved DA, (display authority) is mandatory.

I have always maintained an 'open' view on all this and perhaps a few comments from out there would be more than interesting.

Dennis Kenyon.

.

Brilliant Stuff

18th Sep 2005, 10:30

Dennis Kenyon,

Thank you very much indeed for your detailed explanation.

I bow my head in awe!!

Sadly I have never seen you live but I was given a video of one of your displays and it was a thing of beauty.

Everone who has added they 2 penny's thank you !

N Arslow

18th Sep 2005, 10:38

All sounds terribly straight forward - now try winching while inverted...

widgeon

18th Sep 2005, 11:40

if you auto rotate inverted do you gain altitude :O

Heliport

18th Sep 2005, 13:05

Good to see you in the forum Dennis.

Hope we'll be reading many more of your posts now that you'll have a bit more spare time.

Heliport

NickLappos

18th Sep 2005, 14:18

Having flown a display or two, I also welcome Dennis to the forum!!

The concept of inverted flight has two components, one while holding positive G, and one where the negative G is a trimmed condition.

Any helicopter can be made to show the first positive G condition, if for a brief period. The second requires a very stiff rotor head, stiff blades and a skilled pilot. Even so, no helicopter has the fuel and oil systems to support other than a brief period of negative G. Engine flameout and engine/transmission damage are quite likely if more than about 5 to 10 seconds of negative G are held.

Other than for display purposes, inverted flight is relatively useless, of course. Even airplanes find it better to roll inverted and then pull positive G when a push-down is needed.

Matthew Parsons

18th Sep 2005, 15:01

I haven't flown any attack helicopter, but I seem to recall an Apache pilot telling me that negative g pushover of as much as -2g was used to terrain follow when the terrain was too steep for collective climbs/descents.

A far cry from flying the trimmed negative g that Nick mentioned, but aerodynamically, if you can reach -2g upright you should be able to maintain -1g inverted. Of course, the -2g period would be a constantly changing pitch attitude, so the inverted case would be of a limited duration.

Matthew.

Dave_Jackson

18th Sep 2005, 18:21

Aesir,
The Navy says that flying inverted will result in significant cost savings. They believe that this action this will scare the enemy off without have to use expensive missiles. It's called the 'Top Gun' effect and it was developed by the renowned scientologist Mr. Tom Cruise.

Ascend Charlie,
Lower Rear Admiral Ican B. Bought (nickname 'Rectum') did not realize the extra cost of aerobatics engines. Wanting to continue the tradition of saving the taxpayer's money, he has stipulated that inverted flight will only be done during autorotations.

DennisK

18th Sep 2005, 19:27

Hallo Widgeon.... did you really fly one of those ??

As for inverted 'autorotation' and gaining height. The answer is a definite YES.

After one particular practice, I recall only too well, my old boss, Roy Spooner, (spoonair and all that) expressing concern at how I would cope if the engine let go when I was inverted. With a half tongue in cheek, I replied, "Not a lot extra to do Roy, as often I've split the needles anyway." I don't think he passed out.

But seriously, if a tad too much aft cyclic is used at the top, and with the lever down, the needles will certainly split in an Enstrom or the Schweizer. And when I've had a moment to check the VSI, one IS gaining height, though I've never confirmed this with a positive increase on the altimeter.

Thank you Nick and others for your welcome to pprune. Yes, now I've slowed down and am planning to write in my retirement, I'd love to join you chaps with a contribution or two.

And Mr kissmysquirrel, I AM looking to locate suitable interested younger pilots with a view to producing a 'duo' display, but it will have to be sponsored, or my plan can't go anywhere.

This winter, I plan to attack likely firms. If/when I get a positive response, I'll be looking more actively for three pilots with a view to forming a display team duo for 2007. I have one good guy now and we would want two more, and perhaps a year to get things in order.

The team would be 100% professional, ie full time pilots, dedicated aircraft in house colours, engineer, and ops. Preferred pilots would need a fair level of fixed wing aerobatic experience and perhaps 1000 hours or so on rotary, but nothing needs to be set in rock. But tons of the right mental approach is necessary.

'Twill all be in pprune if/when it can happen.

Dennis Kenyon

NickLappos

18th Sep 2005, 22:24

Matthew,
Dont for a second beleive -2G in an Apache. It has a 4% hinge offset rotor, which will lose roll control at about -1 G and its blades are certainly not capable of producing 40,000 lbs of negative thrust. Chalk that tale up to too much Budwiser and too few ethics!

I certainly can believe zero or even -1/2G, though.

Remember, at zero G the aircraft is making the same radius downward that a 2G pullup produces upward. Each case has 1G of maneuver being performed from the trimmed 1G level.

3top

19th Sep 2005, 01:57

Hi all!

Nick, Dennis,

you are the specialists on this thread:

How does one get a display permit??

I remember the discussion about the chap in South Africa who looped and rolled the 407, though it seems without Bell's and the CAA's "agreement".
It would seem to me that NO one will accept any responsibility IF the sought after maneuvers are not in the POH.
What manufacturer would - today - still agree to authorize such routine done in their product?

Dennis how did you manage that with Schweizer ( ....love the video with the backflip!)?

When I first time heard about the 407 routine in SA, I thought WOW, now Bell has something to shut up everyone who badmouthes the 407 for its "turbulent" infancy ( ...me included)!

However they didn't really appreciate the effort, did they!!!

On the other hand the Aussy Army did routines regulary at airshows rolling/looping A-Stars on regular inspections/maitenance and supposedly they are still flying today!

Can't imagine Frank to give anyone a "go ahead" for some fun in a R-22/44 either!

So, how did you ever manage to get the OKAY from CAA/manufacturer?

3top :D :cool:

DennisK

19th Sep 2005, 21:00

For 3top,

The DA, (display authority) is straightforward. You make contact with the GAD at CAA, either Rob Metcalfe, Dave Evans or Keith Thompson.

Advise you want the DA. Providing they consider your experience adequate, you will be given a DA application and assessment form. You bring that to me or a rotary DAE (display evaluator)

We chat through your proposed display routine and your associated display attitude and knowledge of 403, (display manual.) Next you demonstrate the routine as discussed. No heroics, no special wheezes, just a simple and straightforward display routine that sticks exactly to the planned brief.

If all satisfactory, the DAE forwards his recommendation to the CAA for issue of a DA or otherewise. Just bear in mind, that even when received, your DA will say "Flypasts Only !!!!!!!!!!! "

That's the approval aspect.

To move on to the manoeuvres. All FM's state "no aerobatic manoeuvres" So what are aerobatics? The CAA will advise.... any manoeuvre that exceeds 90 degrees in roll or 60 degrees in pitch.

I have flown my standard display routine before the Enstrom factory officials and have their outline blessing for the manoeuvres flown.

Insurance needs to be tackled next. My insurers, have added to my policy. Occasional display flying by the insured only.

God bless with your efforts.

Dennis Kenyon

tall and tasty

19th Sep 2005, 21:51

DennisK

I was recommended your display by a fellow collegue in the company I have joined and watched your display this year at Shoreham for the first time and it was poetry in motion.

To be honest I was stunned watching it live, I have seen it on video but to not a patch on the real thing.

Thank you so much, but now I am saddened I may not get a chance to watch you again live

TnT

Matthew Parsons

20th Sep 2005, 12:25

Nick, time spent in a recce is never wasted. I'm certain I'll remember that in advance one of these days.

Google search reveals -0.5g to +3.5g and the Apache pilot sitting beside me recalled -1.2g, which was never used. BTW, it wasn't Bud, it was Moosehead.

Matthew.

Brilliant Stuff

22nd Sep 2005, 08:57

Dennis K

I wish you the very best of Success/Luck to get your professional helicopter display up and running!!!

To borrow an american saying "THAT WOULD BE AWESOME"

I wish I could help you out but I don't fly fixed wing and I think there will be about a 100 more qualified people in front of me who are even mader then I am.

Looking forward to your TEAM in 2008.

I would have thought this would be right up RED BULL's street for sponsoring something as excellent as that.

DennisK

23rd Sep 2005, 08:20

For Mr Brilliant Stuff

Thanks for the contribution.... and possible 'Red Bull' involvement.

I did approach their publicity guy last year, but instead they went for the Vixen in dashing colours!

For a display team, (if/when I can get it up and running) nothing is set in rock. I'm far from being 'you must have experience nut' as in my view, type handling skills and mental approach are the more important ingredients to safe display flying.

Over the years, the problem I have experienced, is spreading my display routine along the crowd line. At RAF Cosford it took a minute S & L to do it! Hardly interesting.

So a 'duo' synchronised display would change that. Each manoeuvre R/T triggered as usual, 'Reds' cross overs et al, and I suppose smoke and perhaps booby cheer leaders handing out the sponsors publicity.

I believe we do need more 'novelty' items in the display scene. My '1000 tennis balls drop from 1000 feet' into a target dustbin, * at the RAFA Shoreham event a couple of weeks ago, was particularly well received. (and raised £1.530 for my scholarship)

Anyone with the right sponsor 'contacts' out there please e-mail me.

Dennis Kenyon.

* PS We missed the dustbin !!|

**And we had Article 56 in place.

Gazelle2

31st Jan 2006, 11:53

Nick,

Thanks, your answer brings back happy memories of the Gazelle too (which I haven't flown for 10 years or so now); although not being a test pilot I always managed to keep mine in PERFECTLY balanced flight (LOL), apart from the odd roll off the top where the ball sometimes got half a width out ;) ).

Teetering,

Not forgetting Lofty Marshall of course! :) .

I understand Ray Lawrence is sadly no longer wth us. What genius that modest man had. I got on very well with him as a student and was privileged that in a quiet moment he once showed me his photo album of his inventions / projects, mostly from the 1960s. They were incredibly good and they all worked as intended. He apparently seldom showed them off as he did not like to blow his own trumpet.

For example:

His man-carrying helicopter powered by a 500cc Triumph motorcycle engine.

A Triplane designed, built and flown by himself in Borneo.

His (large!) radio controlled model helicopter with all the mechanicals and radio gear made by himself. The one-piece flywheel / centrifugal clutch was seen in the 1960s and copied by a German enthusiast who now manufactures them (sadly Ray never patented it, which he should have done).

Model 3-cylinder and a 5-cylinder radial aero engines, all of his own design and manufactured on a lathe.

A 5-inch reflecting telescope; the mirror and lenses made by himself.

A hovercraft. A caravan. A speedboat.

All his own design and made by his own hands. Sadly missed by me, he was a great inspiration at the beginning of my career, not forgetting his clever explanations of things aerodynamic and rotating at CFS(H).

Wherever you are, Ray, thanks and good on yer!

Sorry for getting well off track there, folks :rolleyes:

ShyTorque,

Sorry that it took more than a year to answer your posting but I just found this forum yesterday.

I enjoyed your summary of a few of the things that Ray Lawrence did during his active RAF days while teaching all of us hotshot rotorheads how rotary-wing aerodynamics really worked.

The only thing I found wrong about your post was that you thought that Ray Lawrence was "no longer with us." I haven't checked in the last month or so but as of the first of this year Ray was still around.

I stay in touch with Denis Herrett who was OC Standards at CFS(H) when I was there from 1973-1976. Denis and Margarethe moved to Australia in the mid-80s and are still thriving there. Denis stays in contact via email and internet phone with several of the old CFS(H) crowd including Ray and Sheila Lawrence, Ron and Jill Cunningham, and Alan and Carol MacGregor. If you wish to contact me via the email address in my profile, I would be more than willing to share any of their email addresses with you.

It was great to see the name Lofty Marshall again. If there was ever a character who showed what a benefit the RAF gained from the Specialist Aircrew scheme, it was Lofty.

I still exchange Christmas cards with Roy Garwood and Charlie Parsons. Roy was the OC CFS(H) when I went through as a student in 1973. Charlie owned a pub in Anglessey for about 20 years but is now back as a flight instructor at Shawbury.

The only guys from that period at Ternhill who I know for certain are no longer with us are my good friends Tim Seabrook, George McCracken, and Eric Shelmerdine.

I also apologize for taking this thread so far afield.

Gazelle2 (Phil Stinson, USAF - Retired)

funfinn2000

2nd Feb 2006, 05:44

So I guess thats a no then?

eagle 86

2nd Feb 2006, 06:30

Aaaah - the memories - I clearly remember many of those mentioned as I went through CFS(H) late '72 - glad to hear that most are still around - I thought they were old blokes then!!
GAGS
E86

Hamna

2nd Feb 2006, 17:31

Hey Guys....of course a helicopter can fly inverted. Just look at the bell 407 at Virginia Airshow Durban South Africa, 2 years ago. The pilot did a loop as well as a straight roll for the crowd. Needless to say the aircraft was then and still is grounded until about $1000 000 has been spent on getting it airworthy and signed out again..........

funfinn2000

2nd Feb 2006, 19:47

yeah that was'nt sustained inverted flight ,

White Cloud

3rd Feb 2006, 02:34

It was also a pathetic roll and a half-arsed loop. I saw the Tiger and an Apache do loops and rolls at an airshow and the success of their loops and rolls were never seriously in doubt. Inverted! Not sustained, but both came to a hover and just rolled inverted for several seconds before pulling through. Impressive.

rudestuff

22nd Sep 2006, 22:08

anyone have experience of the MIL 34? Piston engine, and supposedly fully aerobatic...

soupisgoodfood

5th Sep 2008, 16:58

"RC models have no teetering hinge normally, so the head is rigid (hingeless) except for the blade attach bolt that also serves as a lead lag hinge."

Not quite right. Most RC heli heads have o-rings between the hub and the spindle shaft that holds the blade grips. Using different o-rings allows you to adjust the stiffness. A common mistake is to set the heli up so that it is soft, then trying some hard 3D stunts, only to suffer a boom-strike. There are many other types of hubs out there, but most use the above setup. And some people do even setup rigged heads, but it's not common -- not even for the top 3D pilots.

The main reason RC helis can hover upside-down with no problems is basically down to this: Scales of physics, or whatever you want to call it, means that RC helis are generally very strong and ridged for their size and stress on components is not such a huge problem when the designer is balancing the engineering with the flight envelope. And because of that, it's a simple adjustment to allow some negative pitch, and you can fly upside-down all day long and do no more harm to the mechanics than if you had been flying the more conventional way.